... Although a lot can be said for the robustness, clean organization, and stability of the BSD operating systems, security is one of the main reasons system administrators use these two platforms. There are plenty of books to help you get a FreeBSD or OpenBSD system off the ground, and all of them touch on security to some extent, usually dedicating a chapter to the subject. But, as security is commonly named as the key concern for today's system administrators, a single chapter on the subject can't provide the depth of information you need to keep your systems secure. FreeBSD and OpenBSD are rife with security "building blocks" that you can put to use, and Mastering FreeBSD and OpenBSD Security shows you how. Both operating systems have kernel options and filesystem features that go well beyond traditional Unix permissions and controls. This power and flexibility is valuable, but the colossal range of possibilities need to be tackled one step at a time. This book walks you through the installation of a hardened operating system, the installation and configuration of critical services, and ongoing maintenance of your FreeBSD and OpenBSD systems...

This book is on the pricey side (IMO), so I picked up a copy at my local uni library. As usual, you get what you pay for; for relatively savvy sysadmin types, this book will take your BSD-focused security knowledge to the next level.

I've worked a fair amount with FBSD jails, and still found some very useful tips within. Another valuable lesson from this book was a proper strategy for utilizing file flags and securelevels. HIDS, packet filtering, service hardening, etc. are covered in detail.

I own and have read several different Linux-focused security books, but this is the most comprehensive BSD-focused security title I am aware of. Definitely worth a read. (And if you're familiar with both FreeBSD and OpenBSD you will likely get even more value from it.)